JP2008-14431A discloses a shock absorber that is installed in a front fork of a motorcycle to perform damping force adjustment using a motor. The shock absorber includes a shock absorber main body having a cylinder coupled to an outer tube, a piston that is inserted in the cylinder to be free to slide and divides an interior of the cylinder into a contraction side chamber and an expansion side chamber, and a piston rod that is inserted into the cylinder such that one end thereof is coupled to an inner tube, which is inserted into the outer tube to be free to slide, and another end is coupled to the piston. The shock absorber further includes a passage connecting the contraction side chamber and the expansion side chamber of the shock absorber main body, a check valve provided midway in the passage to allow only a flow traveling from the contraction side chamber toward the expansion side chamber or conversely to allow only a flow traveling from the expansion side chamber toward the contraction side chamber, a needle valve provided midway in the passage, and a stepper motor that is fixed to the other end side of the piston rod in order to drive the needle valve.
During expansion, the shock absorber generates damping force by applying resistance to a flow of working oil using a piston valve provided in the piston, and during contraction, the shock absorber generates damping force by applying resistance to a flow of working oil flowing out of the cylinder into a reservoir using a base valve provided in an end portion of the cylinder.
Further, in this shock absorber, the working oil is caused to flow through the passage either only during expansion or only during contraction by an action of the check valve. By having the needle valve apply resistance to this flow of working oil, the needle valve generates damping force cooperatively during either expansion or contraction of the shock absorber. Furthermore, by driving the needle valve using the motor, the damping force generated by the needle valve can be varied.
Hence, the needle valve performs a damping function only during expansion or contraction of the shock absorber. A front fork that straddles a vehicle wheel of a motorcycle typically straddles the vehicle wheel in a left-right pair. Therefore, the needle valve of the shock absorber built into one front fork performs the damping function during expansion, and the needle valve of the shock absorber built into the other front fork performs the damping function during contraction. As a result, the expansion side damping force and the contraction side damping force can be adjusted by the front fork as a whole.
According to this shock absorber, the flow of working oil passing through the needle valve always travels in one direction and is therefore a stable flow. As a result, the damping force generated by the shock absorber can be adjusted accurately.